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BIG PHYSICS, BIG QUESTIONS –

Science: Did the AIDS virus originate in chimpanzees?

By PHYLLIDA BROWN

VIRUS found in a chimpanzee in Gabon has reopened the debate about the
origins of AIDS. Researchers in Gabon and France have sequenced the genetic
material of the virus and discovered that it has more in common with HIV-1
than any other immunodeficiency virus found in monkeys. HIV-1 is the more
common of the two human viruses that cause AIDS.

There are two possible theories to explain the AIDS epidemic: either
that HIV has always been present in the human population, but has until
recently remained isolated or unnoticed; or that it ‘jumped’ between species.

In the past, researchers have found viruses from the same family as
HIV, the lentiviruses, in several species of primate other than humans –
African green monkeys, mangabeys, mandrills and captive macaques. But claims
that the virus could have crossed from primates to infect humans in recent
history have been largely discounted, mainly because the so-called simian
immunodeficiency viruses (SIVs) are only distantly related to HIV-1.

The discovery of a chimpanzee virus that closely resembles the human
one means that we cannot, now, discount the theory that animals were the
source of the original infection, say the researchers. But neither can we
go so far as to conclude that the chimpanzee virus was the precursor of
the human one, nor that it somehow travelled from chimps to people.

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‘If you suspect HIV came from animals, you can’t use that to say that
this is the origin of the current epidemic,’ says Simon Wain-Hobson, one
of the researchers. He says it is more likely that the epidemic spread from
a small, isolated human population where the virus had always been present
– as a result of demographic and social change.

Georges Roelants at the International Centre for Medical Research in
Franceville, Gabon, first discovered the virus in two wild chimps in 1988.
The team obtained a sample of it from one of the animals, and, working with
Thierry Huet and his colleagues at the Pasteur Institute in Paris, sequenced
the genetic material of the virus and compared it with that of the known
strains of HIV-1, HIV-2 and three simian viruses (This Week, 24 February
1990). Their results are now published in Nature (vol 345, p 356).

They found that the virus, which they called SIVCPZ, had the same overall
genetic organisation as HIV-1: unlike all the other simian viruses, its
genome includes all the HIV genes – gag, pol, vif, vpr, tat, rev, vpu, env
and nef.

In addition, the amino acids in SIVCPZ’s genetic material are arranged
in strikingly similar fashion to those of HIV-1. In particular, the amino
acids in the gag and pol genes match their counterparts in HIV for 75 and
85 per cent of their respective sequences. By comparison, the simian viruses
match HIV for between 52 and 60 per cent of the sequences in these two genes.

Despite all the similarities, there is more variation between the chimp
virus and all the strains of HIV-1 than there is variation between these
strains, says the team.